Maximum preservation of visual function is the main goal of glaucoma research. A patient cares little about his or her intraocular pressure or optic disc topography so long as their sight is normal. It may thus seem surprising that virtually nothing in known about the effect of glaucoma on the physiological properties of retinal ganglion cells, the very cells that provide the brain with visual input and that are targeted by the disease. The objective of the research project in to fill this important gap in our understanding of glaucoma. The research project proposes to do so by recording in vivo the visually-evoked activity of single optic tract fibers in the rat model of pressure-induced glaucoma and putting directly to test the hypothesis that the functional characteristics of retinal ganglion cells change in chronically hypertensive eyes before the cells die. The specific aims are to quantitatively characterize: 1) the spatiotemporal response properties of ganglion cells in normal rats and 2) the spatiotemporal response properties of ganglion cells in rats with chronic ocular hypertension. One possible outcome of the research is that glaucoma has no ill effects on ganglion cell physiology. For all intensive purposes the cells behave normally up until the moment they receive the apoptotic signal to die. This would dampen the prospects of using non-invasive methods, like vision tests and electroretinograms, for glaucoma detection since cells would invariably have to die for these methods to detect the disease. The other possible outcome is that ocular hypertension alters the mean spike rate, receptive field size, or other response properties of surviving ganglion cells. The physiological effects might be confined to certain types of ganglion cell or shared by them all. This outcome would not only paint a more promising picture for early glaucoma detection but also suggest the kinds of vision test that would best reveal perceptual deficits, which would thereby help speed progress in the fight against the debilitating disease. Public Health: Glaucoma is an eye disease that kills the cells that provide visual input to the brain. The aim of this research is determine how glaucoma affects the neural messages of these cells while the cells are still alive. This information could be used to design novel vision tests that detect the disease at an early stage.